(385v) Reactive Molecular Dynamics-Guided Deconstruction of Single-Use Plastics for Value-Added Chemicals and Novel Materials

The growing environmental concern surrounding single-use plastics necessitates innovative approaches for their sustainable management. In this study, we investigate the catalytic deconstruction of plasma-treated single-use plastics, aiming to transform them into value-added chemicals and novel materials. Leveraging molecular dynamics simulations, we elucidate the underlying mechanisms of plastic breakdown at the atomic level. Our findings provide crucial insights into designing efficient catalysts, considering reaction conditions, and predicting the formation of novel materials during the deconstruction process. This interdisciplinary approach bridges materials science, catalysis engineering, and computational modeling, offering a promising avenue for addressing plastic waste challenges. Among different single-use plastics, polyethylene terephthalate (PET) is the most recycled, but other plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) contribute significantly to pollution due to difficulties in recycling, often ending up in landfills or aquatic environments. Our research focuses on upcycling these challenging-to-recycle plastics like PE, PP, and PS.

In our research, we investigate the breakdown mechanism of polymer chains using ab initio molecular dynamics. Subsequently, we employ reactive molecular dynamics (RMD) to predict all possible products under varying reaction conditions. Specifically, we simulate the plasma treatment process, considering potential free radical reactions. For instance, oxygen plasma can generate ozone as a reactive species. This ozone then reacts with hydrocarbons in plastics, leading to the formation of additional radicals such as H*, OH*, and HO2*. Our ab-initio and RMD simulations provide evidence for these processes. We conducted Ab Initio Molecular Dynamics (AIMD) simulations using the Vienna Ab Initio Simulation Package (VASP) to obtain accurate electronic structure information. These simulations account for quantum mechanical effects, providing insights into the behavior of polypropylene breakdown and product formation at the atomic level. From AIMD trajectories, we extract local configurations around polypropylene molecules, which serve as training data for the ReaxFF force field. By interpolating between known AIMD results, we learn the force field parameters, significantly reducing computational costs compared to fully quantum-mechanical simulations. To validate the ReaxFF, we compare its predictions with additional AIMD simulations on plasma-treated plastics in water medium. The force field’s accuracy is assessed through thermodynamic properties, vibrational spectra, and structural features. Finally, we successfully compare the predicted results with experimental observations. Beyond deconstruction, the ReaxFF enables predictions of novel materials that may form during the breakdown of plasma-treated polypropylene, providing valuable insights for designing sustainable alternatives and recycling strategies.

In a groundbreaking contribution to the literature, we present the -C-C- polymer breakdown mechanism captured through AIMD study. The process begins with an activated ozone radical (O-atom) derived from an ozone molecule. Surprisingly, despite its proximity to a primary hydrogen (H), the O-atom targets the secondary hydrogen (secondary-H), which possesses a C-H bond 4 kcal/mole weaker. Through a multistep sequence, this H-atom is transferred to another -O radical, resulting in the formation of an -OH radical. Subsequently, the -O radical attaches to the secondary carbon, leading to the formation of a -C=O bond, ultimately breaking the C-C backbone. The other end of the broken iPP chain has its H-atom react with a free -O radical to form H2C=CH-. The breakdown process leads to chain scission, resulting in a reduction in the molecular weight of the polymer. Smaller polymer fragments are more susceptible to further degradation. During the breakdown process, reactive species (such as H*, OH*, and HO₂* radicals) are generated. These species can further participate in chemical reactions, leading to the eventual degradation of the plastic. We report here the formation of C2-C4 chains and oxygenated intermediates like ethers and ketones along with HCHO and HCOOH, but these products were dominated by small gas phase products such as CH₄, H₂O, H₂, CO, CO_2. In summary, understanding polymer breakdown and small molecule formation mechanisms guides sustainable alternatives and recycling strategies, contributing to a circular economy.